The present invention relates to a method of processing food. In particular, the present invention relates to a method of processing food by means of exposure to a selected band of infrared radiation which heats an interior of the food without substantially browning the exterior surfaces.
The term "processing" for purposes of this disclosure includes all forms of cooking including: baking, thawing, proofing, deep heating, and selectively thawing, for example.
The use of infrared radiation to cook dough is known. "Infrared Radiation" hereinafter referred to as IR radiation, for purposes of this disclosure includes electromagnetic radiation in the wavelength range from 760 to 10,000 nanometers. The use of IR radiation to cook foods offers several advantages over conventional baking.
Devices which emit IR radiation do not heat the air in the oven between the radiator and the product to be baked. Better process control is possible by using IR radiation to cook food, as compared to conventional baking.
Baking times can be faster with short wave radiation as compared to conventional baking. For products of less than a centimeter in thickness, short wave radiation generally cooks foods faster than conventional baking. A portion of the radiation in the IR wavelength range penetrates the surface of the food to be cooked, and heats the interior of the food. In contrast, with conventional cooking, the exterior surfaces of the food are heated, and the remainder of the food is heated by means of conduction from the exterior surfaces. Consequently, food baked by means of IR radiation typically reaches the selected final temperature faster than baking in a conventional oven.
It has been found that dough based products baked by means of IR radiation have better textural qualities, have a thinner crust and a finer crumb structure which are characteristics that are desirable to consumers.
It is known in the art that when making a determination as to whether to use IR radiation to heat food, it is necessary to first determine which wavelength band is most efficiently absorbed by food being heated. The selection of the wavelength band depends upon the infrared characteristics of the material being heated. For example, the deepest heating beneath the surface for bread crumb and crust occurs with radiation in a wavelength range between about 800 and about 1250 nanometers. A penetration depth of 3.8 millimeters for crumb and 2.5 millimeters for crust was measured at a wavelength band between 800 and 1,250 nanometers. C. Skjoldebrand et al., Optical Properties of Bread in the Near-Infrared Range, 8 J. Food Engineering (1988), pages 129, 137.
Once the favorable wavelength range to obtain the desired results has been determined, a source with a temperature giving a peak output in this range is selected. For a blackbody radiator which operates at about a 3200K source temperature, the wavelength spectrum includes radiation between about 300 nanometers and extends beyond 4,000 nanometers. Only the fraction of the radiation which falls into the range between 800 and 1,300 nanometers is effective in heating beneath the food surface. It is estimated that at best, only about 35% of radiation emitted from a 3,200K source radiator actually serves the purpose of efficiently deep heating the food product. The remaining radiation heats the surface of the food and results in browning or is reflected. Depending on the results desired, browning may be undesirable. For example, if the object of heating is to cook a bakery product which the consumer browns at home shortly before serving the product, then it is undesirable to brown concurrently with deep heating during manufacture.
A quartz halogen bulb with a tungsten filament has been determined to deliver at most 35% of its radiation between 800 and 1300 nanometers, and has a peak intensity of about 1,000 nanometers. The balance of the radiation is either below 800 nanometers, or above 1300 nanometers.
Exposing dough to IR radiation from a source such as a quartz halogen lamp with a tungsten filament deep heats the interior of the dough products most efficiently with radiation between about 800 and about 1,300 nanometers. Radiation at wavelengths longer than 1,300 nanometers heats the surface of the dough products. If the processing technique includes completely baking a dough product such as bread, the surfaces of the bread become browned by the time the product is completely cooked.
Before the present invention, food processes using IR radiators have been controlled by changing the source temperature which shifts radiant power and the wavelength distribution curve. Upon lowering source temperature, a lesser amount of heating beneath the dough product surface occurs and a relatively greater amount of surface heating occurs. When less surface heating is desired, the temperature of the radiator is increased. A relatively greater portion of shorter wavelength radiation is delivered which is more capable of penetrating into the dough product.
Heated bodies which are the source of infrared radiation radiate energy simultaneously over a wide range of wavelengths. Adjusting the temperature of the source provides only limited control of the fraction of the total power radiated in the 800 to 1,300 nanometer deep heating wavelength band. At best, about 35% of the total power radiated lies in this band.
For this reason, controlling both surface heating and deep heating of a food product by means of changing the source temperature of the radiator does not provide for good process control as the surface of the food product is heated and browned before the interior is sufficiently heated.
Although IR radiation has before the present invention been a valuable method of heating dough-based and other food products, its use is limited to processes which require that the final product be surface browned.
The use of IR radiation to proof dough has been described in the art. Katz U.S. Pat. No. 4,917,914 describes a method of proofing dough which includes the process step of exposing 0.63 centimeter thick dough pieces prior to being placed in cans to high intensity radiant heaters with current density [sic] up to 20 Watts/inch.sup.2. It is assumed that the author was referring to "watt density", or "total radiant exitance", rather than current density. Using the Stefan-Boltzmann law, and Planck's law, the source spectrum can be estimated.
At 20 watts per square inch, the heater temperature and wavelength of the radiator used to generate the data in the Katz 4,917,914 reference, assuming blackbody radiation, has a source temperature around 870K and has a peak wavelength at about 3,300 nanometers.
Even if the selected radiator did not emit blackbody radiation, the maximum intensity would still be in the same range. For anodized aluminum, the maximum intensity was calculated to occur at about 2,800 nanometers. For oxidized steel, the maximum intensity occurred at about 3,200 nanometers, and with Incoloy 800.TM., the maximum intensity occurred at about 3,230 nanometers. For each of these sources, most of the radiated power lies between about 1,500 to 10,000 nanometers.
From published data, water is highly absorbing in this spectral long wavelength range and so only surface heating occurs. Radiation in this range is known to brown the outer surfaces of the dough during proofing, which is undesirable in a refrigerated dough product.
Devices which simultaneously emit IR and visible radiation are common. Westerberg et al. U.S. Pat. No. 5,036,179 describes an oven employing tungsten light bulbs or arc lamps capable of producing 1,500 watts of radiant energy in the 400-4,500 nanometer range with a peak intensity at about 1,000 nanometers. This type of radiator effectively cooks and browns food in less time than when using IR radiation alone, or when using convection or conduction ovens.
A significant portion of the spectrum described in the Westerberg 5,036,179 reference is in the range of 400 to 700 nanometers. This reference describes the use of a quartz halogen lamp producing 10% of the output in the 400-700 nanometer range.
Katz U.S. Pat. No. 4,792,456 describes a similar method except that the dough is sealed in the containers prior to heating, and a "no-convection" tunnel or a jet sweep oven is used to heat the cans until an internal dough temperature of approximately 302K is achieved.
The use of IR radiation to bake biscuits is also known. U.K. Patent 2,147,787 to Wade describes the use of IR radiation having an intensity peak between 720 and 2,000 nanometers to bake biscuits. IR radiation is used to completely bake the biscuits. The biscuits are then rapidly cooled and packaged. Biscuits prepared in this manner do not experience spontaneous breakage after cooling and packaging. Since the biscuits are fully baked according to the described process, browning must occur upon exposure to the selected radiation source.
In U.K. Patent 2,147,789 B to Wade, a method of biscuit manufacture is described which includes exposing biscuit dough to grey body radiation within the wavelength range of 720 to 2,000 nanometers, the wavelength band having a maximum intensity at a wavelength which is not preferentially absorbed by water. Preferred maximum intensity is between 1,000 and 1,200 nanometers. Such exposure completely cooks the biscuit and browning occurs due to the presence of radiation in the 2,000 nanometer wavelength range.
With both U.K. Patents 2,147,788 B and 2,147,789, the radiators described are broadband grey body radiators whose temperature is chosen to produce maximum intensity between 800 and 1,500 nanometers. However, only about 45% of the energy delivered is in the preferred wavelength band. Heaters with temperatures in the 1,923K to 3,673K range meet these requirements.
The use of IR radiation to bake bread loaves has also been described in Wells U.S. Pat. No. 2,340,354. Drying lamps are heated to a temperature at which the filaments are incandescent and will emit infrared or combined infrared and other rays. Page 1, col. 2, lines 24-30. Exposing dough to such a radiation source completely cooks the dough product, including forming a brown crust.
Devices which utilize halogenated lamps as a source of I.R. radiation are known. U.K. Patent Applications 2 144,956A and 2 132,060A show a cook-top heating device with a plurality of halogenated lamps for delivering IR radiation to foods. The '956A patent describes a radiator having a maximum wavelength of 1,200 nanometers and having a wavelength band within the IR range. The reference describes the use of an optical filter to block out undesirable visible light. Pg. 3, lines 62-65. Visible light has a wavelength range below the IR range.
It is known to use IR radiation to heat prepared food just prior to serving as described in Newkirk U.S. Pat. No. 3,037,443. Prepared foods such as sandwiches are wrapped in material transparent to radiation of wavelengths between 1,500 and 3,000 nanometers and are placed in a device which emits IR radiation in this range. Three minutes of exposure is sufficient to brown the bread and thoroughly heat the entire product.